The present invention relates generally to the field of reinforcing or molding materials impregnated with a synthetic resin (i.e., prepreg materials). More particularly, the present invention relates to an electrically conductive prepreg for suppressing corona discharge in high voltage devices.
Prepreg materials have been used for many years to initially bond inner-cooling channels to high voltage coils prior to the application of a ground wall insulation. Typically, a Dacron-felt material is treated with a thermoset resin to prepare a prepreg material positioned between the inner-cooling channels and coil strips of a high voltage coil. The assembly is heated to cure the prepreg material and effect the bonding of the assembly sufficient to permit its handling. The assembly is then provided with a ground wall insulating cover, usually by wrapping layers of mica tape around the assembly. Because the mica insulation is relatively brittle, the entire assembly is impregnated under pressure with a solventless impregnant. When cured, the impregnant provides the required strength and stability as well as a bond between the ground wall and coil strips.
U.S. Pat. No. 3,960,803, Jun. 1, 1976, titled "Flexible Nontacky Prepreg for Bonding Coils In High Voltage Devices And Method Of Making Said Prepreg," discloses a resin-treated fibrous mat prepreg comprising an epoxy-anhydride resin-loaded DACRON polyester felt, which is process compatible with a vacuum pressure impregnant (i.e., a styrene-epoxy-anhydride impregnant). U.S. Pat. No. 4,112,183, Sept. 5, 1978, titled "Flexible Resin Rich Epoxide-Mica Winding Tape Insulation Containing Organo-Tin Catalysts," discloses a flexible mica winding tape of the type referred to above. These two patents may be referred to for further information on prepreg materials and mica winding tape.
FIG. 1 schematically depicts a high voltage inner-cooled half-coil assembly as disclosed in the above-cited U.S. Pat. No. 3,960,803. As disclosed in the patent, the inner-cooled high voltage coil 10 includes first rows 11 of copper coil strips. The individual coil strips are coated with glass fibers prior to being used in the assembly, and may include an additional coating of insulating varnish or the like. A first prepreg layer 12 comprising a fibrous mat made of glycol ethylene terephthalate polymeric ester fibers treated with epoxy-anhydride resin is positioned between the coil rows 11. In addition, inner-cooling channels 13 and 14 are positioned adjacent coil rows 11, and second layers 16, 17 of epoxy-anhydride fibrous mat prepreg are positioned between the rows of coil strips 11 and the inner-cooling coils 13, 14. A third set of prepreg layers 18, 19 are positioned adjacent inner-cooling channels 13 and 14 as shown. Rows of coil strips 21 and 22 are positioned on the third prepreg layers 18, 19; and a fourth set of prepreg layers 23, 24 are positioned adjacent the coil strips 21 and 22, as shown.
The '803 patent further discloses that the assembled array of coils, channels, and prepreg layers are heated in a bonding furnace to provide an integrally bonded, partially cured assembly. The assembly is subsequently covered with a mica ground wall 26, which is wrapped around the assembled unit to provide the required insulation and protection to the coils. The wrapped assembly is then impregnated under pressure with a styrene-epoxy-anhydride impregnant and heated. The fully cured assembly provides an integrally bonded inner-cooled high voltage coil 10 in which the impregnant is bonded to at least a portion of the prepreg.
However, even with the high degree of process compatibility between the impregnant and resin-loaded felt, the present inventors have discovered that "microvoids" form at the mica/felt interface. These microvoids result in corona and unacceptable loss tangents (tan .delta.) at high voltages.